Converging evidence from diverse experiments suggests that the balance of excitation and inhibition is disrupted in autism with widespread repercussions on neural communication. The goal of the proposed project is to investigate the largely unexplored issue of the integrity of neural elements that underlie excitatory and inhibitor control within prefrontal networks. Experiments will be conducted on post-mortem brain tissue from 3- 10 year old children, using the robustly interconnected anterior cingulate, orbitofrontal, and lateral prefrontal cortices as a model system. These areas have a key role in the processes of attention, emotions and behavioral flexibility, which are consistently affected in autism. The overarching hypothesis is that altered brain connectivity in autism affects in distinct ways short and long-range frontal cortical pathways and local inhibitory neurons, disrupting neural communication and the balance of excitation and inhibition. This hypothesis will be tested by investigating the status of: (1a) excitatory axons in the deep white matter that link prefrontal cortices with distant areas, which are desynchronized in autism; (1b) excitatory axons and their expression of the growth-associated protein GAP-43 in the superficial white matter, which links neighboring prefrontal areas and is enlarged in children with autism; (2) excitatory axons and their expression of GAP-43 in different cortical layers that receive or issue driving feedforward o modulatory feedback pathways in prefrontal cortices in autism, and; (3) the laminar composition and relationships of three functionally distinct neurochemical classes of inhibitory neurons in prefrontal grey matter, which underlie cortical inhibitory control. Data will be used to: (4) computationally model complex interactions of frontal circuits, associated with the integration of attentional and emotional processes, and the ability to flexibly shift attention, which are commonly disrupted across the autism spectrum. The choice of ages 3-10 years aims to capture documented atypical and typical development of the prefrontal cortex of children with autism and controls. Axons will be double-labeled for myelin, and for GAP-43, which is expressed in development and after brain injury, for study at the light, confocal, and electron microscopes, and will be reconstructed in 3D. Inhibitory neurons will be labeled to distinguish functional classes by their neurochemistry and known mode of innervation. Findings will provide a rich quantitative database on the features of excitatory axons and inhibitory neurons in the brains of children with autism to compare with age-matched controls and with available data of persistent changes in axons in the prefrontal white matter of adults with autism. Comparison across ages will help delineate a timeline for the developmental changes in autism, and help design future experiments to study whether early axon growth or persistent inflammation may underlie the pathology. The proposed studies will provide novel data on fine features of neural elements to model the disrupted excitation-inhibition and neural communication in autism. The findings will have important implications for the development of therapeutic interventions in autism.